Transformers are known in the art and they are used in different types of circuits. Miniaturization has allowed these transformers to be integrated on-chip. Such an integrated transformer is typically used at the input of an RF circuit, where it is typically used as a balun (balanced-unbalanced) to convert a single-ended signal coming from an antenna into a differential signal before entering a differential amplifier. When differential signaling is used, the corresponding coil typically has a center tap which is connected to ground, so that one terminal of the coil provides a positive signal, while the other provides a negative signal. Ideally the signals on these terminals are exactly opposite to each other over the frequency range of interest, meaning their amplitude is identical, and their phase difference is 180° over the entire frequency range. In practice of course, this is not exactly the case, but usually a structure with more geometrical symmetry also provides better electrical symmetry. An integrated transformer is typically also used at the output of an RF circuit, where it is typically used as a balun to convert a differential signal coming out of power amplifiers into a single-ended signal to be applied to the antenna. Transformers can also be used to convert a first single-ended signal into a second single-ended signal, of the same or a different voltage depending on the number of turns of the coils. Transformers can also be used in fully differential mode to connect a differential output of a first circuit to a differential input of a next circuit with galvanic separation.
At millimeter-wave frequencies, such an integrated transformer is typically implemented using two layers of one turn transformers. Unfortunately this structure has to be relatively large to obtain a certain inductance. To increase the inductance the area can be increased or the number of turns, or a combination of both. Increasing the number of turns usually means increasing the number of layers. For on-chip transformers however, especially for RF applications, the practical limit of the number of layers is usually limited to two.
Several transformer structures are proposed in the art. U.S. Pat. No. 6,870,457 describes a symmetrical stacked inductor comprising a plurality of conductive layers separated by isolating non-conductive layers, the conductors on the separate layers being interconnected using vias through the isolating layers. A disadvantage of the structures described in U.S. Pat. No. 6,870,457 is that at least three metal layers are required to create a two turn transformer.
U.S. Pat. No. 7,482,904 describes an on-chip transformer balun including a primary winding as an input terminal of the on-chip transformer balun, and a secondary winding as an output terminal of the on-chip transformer balun. A disadvantage of this structure is that the proposed structure having two times two turns cannot be implemented in two conducting layers, as each of the winding structures requires an overpass or underpass.
U.S. Patent Application Publication no. 2003/0137381 describes a transformer balun that is symmetrical in structure and can have any number of turns. A disadvantage of this structure is that a transformer having two times two coils cannot be implemented in two conducting layers, as each coil requires two layers because of the required metal bridge.
European Patent Application Publication no. 0 902 443 describes a planar coupled coil arrangement of two magnetically coupled coils, each coil having two windings, the arrangement being implemented in two conducting planes. A disadvantage of this coil structure however is that the terminals of the two coils are located at the same side of the structure, making it difficult to connect it in a chain.